This invention relates to electromagnetic discharge apparatus and in particular to electrodeless light sources which have a metallic coating on the inner surface of the lamp envelope to protect the lamp envelope from degradation caused either by the lamp fill or by products of the discharge and to act as a catalyst for the recombination reaction of molecular lamp fill material.
electrodeless light sources which operate by coupling high frequency power, typically 915 MHz, to a discharge in an electrodeless lamp have been developed. These light sources typically include a high frequency power source connected to a termination fixture with an inner conductor and an outer conductor surrounding the inner conductor as described in U.S. Pat. No. 3,942,058 issued Mar. 2, 1976 to Haugsjaa et al. and U.S. Pat. No. 3,943,403 issued Mar. 9, 1976 to Haugsjaa et al. The electrodeless lamp is positioned at the end of the inner conductor and acts as a termination load for the fixture. The termination fixture has the function of matching the impedance of the electrodeless lamp during discharge to the output impedance of the high frequency power source. The electrodeless lamp envelope is made of a transparent substance and encloses a fill material which emits light upon breakdown and excitation.
Various other electrodeless discharge devices have been described in the prior art, for example, U.S. Pat. No. 4,010,400 issued Mar. 1, 1977 to Hollister and U.S. Pat. No. 3,787,705 issued Jan. 22, 1974 to Bolin et al., U.S. Pat. No. 3,873,884 issued Mar. 25, 1975 to Gabriel, U.S. Pat. No. 3,872,349 issued Mar. 18, 1975 to Spero et al., and "Microwave Discharge Atom Source for Chemical Lasers", R. A. McFarlane, Rev. Sci. Instrum., Vol. 46, No. 8, August 1975.
Although most electrodeless lamps are filled with an inert gas and envelope-compatible substances such as mercury, it is sometimes desirable to use fill materials which attack the lamp envelope. One example of a material which attacks the lamp envelope is found in low pressure electrodeless deuterium discharges. The deuterium discharge is normally used as an ultra-violet continuum source in spectrophotometers. Because of the reaction between deuterium atoms and the quartz envelope, the life of the light source is low.
Electroded deuterium light sources which are presently in commercial use are unsatisfactory because of poor stability of light output. In addition, the lifetime is on the order of 125 hours as a result of a reaction between the deuterium fill material and both the electrodes and the quartz envelope.
Conventional methods of protecting lamp envelopes from reactive fill materials have emphasized the use of an envelope material which is compatible with the fill material. For example, glasses which contain boric oxide are resistant to sodium attack. Current low-pressure sodium lamps contain an inner liner of borate glass. In the case of high pressure sodium lamps, an alumina envelope is used. It would be desirable to use a common envelope material for many types of discharge lamps. Quartz, for example, is desirable because of its high temperature properties and its ease of sealing and shaping.
In addition to the above-mentioned patents, the following U.S. patents, which may be of interest, relate to electrodeless lamps, at least one of the patentees of each patent is an applicant of this application, and all patents have been assigned to the assignee of the present application.
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 3,942,068 Haugsjaa et al. March 2, 1976 3,943,401 Haugsjaa et al. March 9, 1976 3,943,402 Haugsjaa et al. March 9, 1976 3,943,404 McNeill et al. March 9, 1976 3,993,927 Haugsjaa et al. November 23, 1976 3,995,195 Haugsjaa et al. November 30, 1976 3,997,816 Haugsjaa et al. December 14, 1976 4,001,631 McNeill et al. January 4, 1977 4,001,632 Haugsjaa et al. January 4, 1977 4,002,943 Regan et al. January 11, 1977 4,002,944 McNeill et al. January 11, 1977 4,041,352 McNeill et al. August 9, 1977 4,053,814 Regan et al. October 11, 1977 4,065,701 Haugsjaa et al. December 27, 1977 4,070,603 Regan et al. January 24, 1978 ______________________________________